Teletypewriter subscriber set



Dec. 3, 1963 1'. L. DoKroR ETAL 3,113,176

TELETYPEWRITER SUBSCRIBER SET Filed Sept. 29, 1961 5 Sheets-Sheet 1 Dec. 3, 1963 T. L. DoK-roR l-:rAL

TELETYPEWRITER SUBSCRIBER SET Filed Sept. 29. 1961 5 Sheets-Sheet 2 lumi N Si

ATTORNEY Dec. 3, 1963 T. L. DoKToR ETAL TELETYPEWRITER sUscRIBsR sm:

5 Sheets-Sheel'I 5 Filed Sept. 29, 1961 7.' L. DOKTOR G. PARKER L A. WEBER H. M. ZYDNEV A TTORNEY INVENTORS 3, 1963 T. L. DoKToR ETAL 3,113,176

TELETYPEWRITER suBscRIBER SET Filed sept. 29, 1961 s sneets-shet 4 V, R E Mmmm KKBV b ORMZ T gv om :im L A Y .l/H V1 xz s I VA. Du Y 2:, 2 Tlrviw 3v m N mw N mimo ,W y vn W auv o 2F, NQNJ WQMJ No? (i 0 m U 6E v .28 www QQ S i w 1mm E? ww Sv PU NM1 55x S. v

ATTORNEY Dec. 3, 1963 T. L. DoKToR ETAL 3,113,176

TELETYPEWRITER suBscmBER SET 5 Sheets-Sheet 5 Filed Sept. 29. 1961 A TTORNEI* 3,113,176 TELETYPEWRHER SUBSCRIBER SET Theodore L. Doktor, Flushing, and George Parker, New

York, NSY., and Laurance A. Weber, West Grange, Pili.,

and Herbert M. Zydney, New York, NX., assignors to Rell lelephoue Laboratories, Incorporated, New York,

NX., a corporation of N ew York Filed Sept. 29, 196i, Ser. No. 141,672 13 Claims. (Cl. 17d-3) g This invention relates to semi-automatic data transmission sets and, more particularly, to teletypewriter stations which are arranged to communicate by voice frequency signals over a telephone switching network.

A broad object of this invention is to provide an improved control circuit for data transmission sets.

Another object of this invention is to extend a conventional telephone subscriber line to a teletypewriter station.

A further object of this invention is to provide the interchange of supervisory signals and communication signals between teletypewriter stations by way of a telephone switching network.

in well known telephone switching practices, a telephone subscriber initiates a call by removing the telephone handset and then dialing the appropriate digits designat- 2 ing the desired remote subscriber 'when the switching oflice returns dial tone. With the calling subscriber monitoring the progress of the connection with the handset, the switching network extends the call to the called subscriber line and rings the caled subscriber if the line is not busy. The called subscriber then completes the connection by removing the handset whereupon the subscribers may communicate through the telephone switching network. At the conclusion of the call a subscriber returns the handset to the cradle and the two stations are disconnected.

According to the present invention a subscriber data transmission set, arranged to communicate by voice frequency signals, is connected to a subscriber line extending from a conventional telephone switching otlce. The subscriber set is provided with a dial, a listen-only handset and a ringer whereby a call may be processed through the telephone switching network to a remote subscriber data set. When the called subscriber completes the connection, supervisory connect signals are automatically interchanged to enable the data sets to communicate through the telephone switching network. The call is then concluded by either station sending a disconnect signal which restores the remote data set to the initial condition and disconnects the station.

An additional object of this invention is to activate a data transmission set when a remote data set signals that a connection is complete.

Another object of this invention is to complete a connection to a telephone subscriber line in response to ringing signals received therefrom.

Another object of this invention is to monitor for a connect signal during the calling interval and for a disconnect signal during the conversation interval.

Another object of this invention is to provide means for communicating between teletypewriter stations by voice frequency signals.

Another object of this invention is to communicate in a separate frequency band in each direction.

3,l i3, l 76 Patented Dec. 3, 1963 iCC Another object of this invention is to deactivate a teletypewriter when the remote station disconnects.

A further object of this invention is to disconnect a teletypewriter station at the conclusion of a call.

it is a feature of this invention that supervisory .and data signals be converted to frequency shift signals ma first voice frequency band when originating a call and 1n a second voice frequency band when answering a call. i

It is another feature to detect frequency shift signals in the rst frequency band when the station answers a call and in the second frequency band when the station originates a call.

It is another feature of this invention that a teletypewriter be connected to a telephone subscriber line in response to a conventional telephone ringing signal supplied over the line and that the remote calling teletypewriter subscriber be automatically signalled that a connection has been established.

It is still a further feature of this invention to time teletypewriter marking signals prior to the completion of the connection and time the teletypewriter spacing signals during the communication interval by a common timer.

It is a further feature to send a disconnect signal and restore the subscriber line to the ori-hook condition when the subscriber terminates the call.

It is an additional feature to disable a teletypewriter and disconnect the station from the subscriber line when a disconnect signal is received.

The means for fullilling the foregoing objects and the practical embodiment of the features of this invention will be fully understood from the following description taken in conjunction with the accompanying drawing wherein:

FIGS. 1 to 4, when arranged as shown in FIG. 5, show the details of circuits and equipment which cooperate to form a teletypewriter subscriber station in accordance with this invention; and

FG. 6 illustrates in block form the various equipment and circuits of the station and the manner in which they cooperate.

ln several figures of the drawing, the relay contacts are shown detached from the relay winding. Contacts which are closed when the associated relay is de-energized, known as break contacts, are represented by a single short line perpendicular to the conductor line, while contacts which are closed when the relay is energized, known as make contacts, are represented by two short cross lines diagonally intersecting the conductive line. Each set of relay contacts is identified by the relay core designation together with an individual contact number.

General Description Referring now to FIG. 6, telephone line: dill is shown extending from line circuit 692 of the data set to a conventional telephone switching otlice, not shown. Telephone line 691 is a conventional line for the transmission of voice frequency signals and terminates in the remote telephone switching othce in the same manner as a telephone subscriber line. Included in the data set is teletypewriter 603 which includes keyboard transmitting apparatus and printing telegraph apparatus. Typical arrangements disclosing transmitting and printing apparatus suitable for use in this system are shown, for example, in Patents 2,607,848 granted to W. J. Zenner on August 19, 1952, and 2,505,729 granted to W. l. Zenner on April 25, 1950, respectively.

The data set is provided with attendant set 69d. Attendant sct 664 includes a conventional telephone dial, a bell ringer, and a listen-oniy hand set. Additional apparatus is provided in control and logic circuit 695. This apparatus includes a iirst manually operated key, identied as originating key for placing the data set in condition to initiate a call, hereinafter referred to as the originating mode, a second manually operated key, identified as an answering key, for placing the data set in condition to receive a call, hereinafter referred to as the terminating mode, and a third manually operated key, identified as a clear key, for restoring the data set to the normal on-hook condition.

Each station has a frequency shift modulator generally indicated by block 6158, and a frequency shift discriminator generally indicated by block 609. Each modulator and each discriminator is able to switch to either of two frequency bands, the particular band selected depending upon whether the station is in an originating mode or a terminating mode. The lower frequency band, hereinfatcr described as the F1 band, includes frequencies corresponding to the mark and space signals which frequencies are hereinafter designated marking frequency FIM and spacing frequency FIS. The upper frequency band denoted as the F2 band includes frequencies corresponding to mark and spacing signals, hereinafter described as marking frequency P2M and spacing frequency F25. During data transmission, the originating station transmits signals in the F1 band and receives signals in the F2 band, and, conversely, the terminating station transmits signals in the F2 band and receives signals in the F1 band.

Assuming that a station attendant desires to call a remote station, the attendant operates the originating key. In response to the operation of the originating key, control and logic circuit 665 instructs transfer switch 611, and transfer switch 612, to extend the output of frequency shift modulator 6118 to telephone line 691 by way of an appropriate bandpass filter of filters 614 arranged to pass signals in the F1 band, amplifier 61S, hybrid coil 666 and line circuit 692. Transfer switches 611 and 612 are also instructed to extend telephone line 601 through line circuit 6112, hybrid coil 666, amplifier 6117 and the appropriate filter in bandpass lters 614 to pass signals in the F2 band to the input of limiter 616. Control and logic circuit 6115 also conditions timer 610 to look for a continuous marking signal from discriminator 669 as described hereinafter. In addition, control and logic circuit 635 instructs line circuit 6512, to complete a D.-C. path between the terminals of telephone line 6411, thereby presenting an off-hook signal to the central ofiice.

The switching equipment in the central office will return dial tone in the same manner as a normal telephone call. This dial tone is applied by way of hybrid coil 666 and amplifier 667 to the listen-only hand set in attendant set 6114i. When the attendant hears the dial tone she will dial the number of the desired remote station causing dialing pulses to be applied to telephone line 661 by way of line circuit 662. The telephone switching oiice, in response to the dialed digits, performs the conventional switching action to extend the line to the called station in the same manner as an ordinary telephone call. Originating attendant may monitor the progress of the call by listening to the audible tones with the listen-only hand set.

When the call is completed to the called station the telephone oice provides ringing tone to telephone line 6131 of the called station. This ringing tone is applied to the ringer in attendant set 664.

In the event that the called station is arranged for unattended operation, line circuit 662 responds to the ringing signal by informing control and logic circuit 6115 that a call is being received.

If the called station is 4 arranged for attended operation, the attendant will operate the answer key when the station is rung, thereby informing control and logic circuit 695 that the station is being called. 1n either event, control and logic circuit 665 instructs line circuit 6112 to provide an olf-hook signal to the terminating central ofce.

When control and logic circuit 695 of the terminating station is advised that a call has been received, circuit 6515 instructs timer 611i to time an interval of approximately one second. During this interval the terminating station is maintained in a quiescent condition to guarantee the propagation of the off-hook signal from the terminating central oiiice to the originating central office.

At the conclusion of the one second interval, timer 611) times out and control and logic circuit 665, in response thereto, arranges discriminator 609 to receive signals in the F1 band and activates frequency shift modulator 668 to send marking frequency P2M. Frequency shift modulator 6113, in response thereto, transmits marking frequency P2M by way of transfer switch 611, bandpass filter 614, which lters are normally arranged to pass frequency P2M, transfer switch 612, ampliiier 615, hybrid coil 666 and line circuit 662 to telephone line 6111. In addition, control and logic circuit 605 instructs timer 616 to look for a continuous marking signal applied thereto by discriminator 669, as described hereinafter.

At the originating station, marking frequency FQM is received from line 601 and applied by way of bandpass iilter 614 to limiter 616, as previously described. Limiter 616 limits the frequency shift wave and a limited wave is applied to discriminator 6039 which is normally arranged to respond to signais in the F1 band. Accordingly, discriminator 669 applies a marking signal to OR gate 617, and OR gate 617, in turn, applies the marking signal to the input of timer 61@ by way of transfer switch 618.

When a marking signal is received by the originating station for a one second interval, timer 616 times out and control and logic circuit 665, in response thereto, starts up the motor of teletypewriter 693. In addition, control and logic circuit 665 activates frequency shift modulatorv 666 and frequency shift modulator 69S, at the originating station, applies marking frequency FIM to telephone line 6111. Furthermore, control and logic circuit 66S instructs transfer switch 618 to transfer the output of OR gate 617 rom timer 616 to the input of inverter 619 and extend an output of inverter 619 to the input of timer 6111 Timer 610 is, at this time, recycled by control and logic circuit 605. Since the output of discriminator 6119 is now applied to the input of timer 61@ by way of inverter the conditions of the marking and spacing signals are i11- verted, and timer 61@ is thus arranged to look for a continuous spacing signal.

At the `terminating station, marking frequency FIM is received across the telephone line 6111 4and applied by Way of limiter 616 .to discriminator 669. Discriminator' 669, in turn, applies the marking signal to timer 616 by way of OR gate 617 and transfer switch 61S. When the marking signal has been received for an interval of approximately one second, timer 611i times out :and control and logic circuit 605, in response thereto, turns on the motor of teletypewriter 603. IIn addition, control and logic circuit 665 instructs transfer switch 618 to transfer the output of OR gate 617 from timer (16` to the input of inverter 619, and extend an output of inverter 619- to the input of timer 610. Timer 610 is, at this time, recycled `and thus arranged to look for a continuous spacing signal in the same manner as the corresponding timer at the originating station.

In the event that the terminating station is arranged for unattended operation, answer-back generator 626 is. logic circuit 66S, at the termination of the marking interval, energizes generator 626,. thereby providing the application of an answer-back sig provided. Control and nal to frequency shift modulator 6418 by way of break timer 621. If the terminating station is.v arranged for at# tended operation, the attendant thereat operates the keyboard of teletypewriter 6013, whereby an answer-back signal is applied to frequency shift modulator 66S by way of break timer 621.

The stations `are now in condition for communication. Signals transmitted from teletypewriter 663 are applied to frequency shift modulator 668 by way of break timer 621. Frequency shift modulator 608, in response to the marking or spacing signals applied thereto, converts the signals to frequency shift signals in the appropriate frequency band and `applies the signals to the telephone line. Frequency shift signals received from the remote station by way of telephone line 691 are lapplied to the input of discriminator 669 by way of limiter 616. Discriminator 669, in response to the frequency shift signals applied thereto, converts the signals to the conventional telegraph mark and space signals and applies the signals by way of OR gate 617, transfer switch 618, inverter 619 and keyer 622 to the input of the printing apparatus of teletypewriter 663. `It is noted that during the communication interval, timer 61@ at each of the stations is looking for a continuous spacing signal `as previously described.

Break timer 6211 functions to repeat telegraph signals applied thereto to frequency shift modulator 608. In addition, break timer 621 -is arranged to limit the duration of spacing signals to a predetermined interval. This is required in the present arrangement because a spacing signal longer than this interval functions to initiate the disconnection of the station from the telephone line.

The output of break timer 621 is also provided to an input or OR gate 617. Accordingly, signals transmitted from the keyboard of teletypewriter 603 :are passed through break timer 621, OR gate 617, transfer switch 61S, inverter 616; and keyer 622 to the select magnet of the printing apparatus in teletypewriter 603. This provides a local copy of the communication transmitted to the remote station.

When the call is completed, the attendant at one of the stations operates the clear key in the control and logic circuit. Operation of the clear key arranges control and logic circuit 665 to instruct frequency shift modulator 60S to send -a continuous spacing signal in the appropriate frequency band. Control `and logic circuit 695 also turns off the motor `of teletypewriter 603 and recycles timer 616 to time a one second interval.

At the conclusion of the one second interval, timer 610 times out `and control and logic circuit 60S responds thereto by disabling frequency shift modulator 663, restoring telephone line 601 to the on-hook condition and returning transfer switch 611, transfer switch 612, transfer switch 61? `and discriminator 669 to the initial quiescent condition.

At the remote station the one second spacing signal is received by timer 610. Timer 61), which is looking for the yone second spacing signal, as previously described, times out and, in consequence thereof, arranges control and logic circuit 665 in the same manner corresponding to the operation of the clear key. The subsequent functions of the remote station are, therefore, identical to the previously described disconnect functions of the station initiating the disconnect sequence. The one second spacing signal transmitted by the remote station performs no function under these circumstances.

Each of the stations is provided with space-hold detector circuit 623. Space-hold detector 623 functions to observe the level of the frequency shift signals applied to limiter 616. In the event that the incoming frequency shift signal falls below a predetermined threshold level, spacehold detector 623 provides a simulated spacing signal to an input of OR gate 61'7. Accordingly, in the event that the signal is lost because of a transmission failure, for example, a continuous spacing signal is provided by way of OR gate y617 to timer 610 and teletypewriter 663. As a result, timer 6161 times out and arranges control and logic circuit 605 in the same manner as the ope-ration of the clear key. Thus, transmission failure automatically disconnects the two stations.

Break Timer Assuming a connection has been set up as previously described in general terms and the teletypewriter keyboard of the teletypewriter, generally indicated by block 201, FIG. 2, is not active whereby data is not being transmitted, the teletypewriter send contacts 262 and break contacts 203 are closed designating a marking condition. This completes a circuit from negative battery through resistor 12.11B, send contacts 262, break contacts 203, lead 204, resistor 12.13B, HG. 1, and resistor RMB to positive battery applying the negative potential at the junction of resistors R133 and RMB to the righthand plate of capacitor (26B, as shown in FIG. 1. The other plate of capacitor (26B is connected to the base of transistor (24B and when send contacts 262 are maintained in the closed marking condition, transistor Q43 conducts due to .negative battery applied to its base by way of resistor RISB. r1`herefore, negative battery is normally applied through resistor R193', the collector-toemitter path of transistor (24B, the break contacts of test key contacts 106, and the break contacts of relay contacts S-S to the base of transistor QSE.

Assuming now that send contacts 262 open to send a spacing signal, the negative battery applied by way of send contacts 202 and break contacts 263 is removed from capacitor 06B and positive battery is applied thereto by way of resistor RMB. This results in a positive-going voltage spike transmitted through capacitor C613 to the base of transistor Q4B cutting off the transistor. When send contacts 202 reclose to send a marking signal, the negative battery is reapplied to capacitor (26B and the resultant negative-going voltage spike transmitted through capacitor C6B turns transistor Q-/lB back On.

Transistor Q4B constitutes a break timer which responds to a prolonged spacing signal by turning Off for a predetermined maximum interval of time. If, during a conversation, the receiving station wishes to interrupt transmission, a spacing break signal may be transmitted to interrupt the transmitting station. 1t is desirable, however, that this spacing break signal is long enough to be detected at the transmitting station, but sufficiently short not to be misinterpreted as a disconnect signal. Transistor QiB is arranged to turn back On when break contacts 203 are open for a prolonged interval because capacitor C6B discharges by way of resistor RilB to negative battery to reapply negative voltage to the base of transistor QfiB. Accordingly, when a break signal is transmitted by opening break contacts 263, the positivegoing voltage spike through capacitor C63 cuts off transistor (24B, as previously described, but transistor QAtB turns back On after a maximum predetermined duration of time as determined by the discharge time of capacitor C6B whereby no spaces may be transmitted which are longer than the maximum interval.

Modulator Transistor QSB is the transistor which governs the frequency-shift of the data channel. in the quiescent condition, with the teletypewriter marking and transistor QdB conducting, the negative voltage applied to the base of transistor Q3B renders it conductive. When transistor Q3B conducts, and the data set is in the originating mode whereby relay AN is released, as described hereinafter, a conductive path is connected from ground by way of the emitter-to-collector path of transistor QSB, terminals 1 through 3 of inductor T213, the break contacts of relay contacts AN-l and the break contacts 0f contacts AN-Z to terminal 4 of inductor T1B`. Since terminal 1 of inductor TIB is connected to ground, inductor TZB is thus placed in parallel with inductor T1B.

When the data set is in the answer mode of operation the AN relay will be operated and a conducting path from ground is connected by way of the emitterto-col lector path of transistor Q33 and then from terminal 1 to the tap on terminal 2 of inductor TZB, and make contacts of contacts AN-ll and the make contacts of contacts AN-Z to the tap on terminal 3 of inductor TB, placing the portion of inductor TZB between'terminals 1 and 2 in parallel with the portion of inductor TB between terminals 3 and 1.

When transistor Q43 stops conducting, the negative collector voltage of transistor QiB which normally maintains transistor QSB On is removed. Transistor QSB turns Off because of the back bias provided by positive battery to the base of transistor QSB by Way of resistor R163, opening the emitter-to-collector path to terminal l of inductor T213. Accordingly, inductor TZB no longer shunts the tank inductor TTB.

Thus, keyer transistor QSE either places inductor TZB in shunt with tank inductor TIB or removes inductor T213 dependingy upon Whether send contacts 262 or break contacts Qti are closed or open corresponding, respectively, to the mark or space condition to be transmitted.

Transistor stage QZB, FIG. 1, constitutes the oscillator. Its tank circuit consists of inductor TiB and capacitor CdB which is connected between ground and terminal 4 of inductor TIB when relay AN is released or terminal 3 of inductor TiB when relay AN is operated. The basic frequency of this oscillator, when relay AN is released, is controlled to oscillate in the lower or F1 band. If relay AN is operating the amount of inductance across capacitor C43 is reduced, as previously described, resulting in a basic frequency in the higher or F2 band. In addition, when send contacts 262 are closed, corresponding to a marking signal, transistor QSB is conducting and the oscillator tank circuit is shunted by inductor T2B, as previously described, reducing the inductance across the tank circuit and causing the oscillator to oscillate at a higher frequency in the basic frequency band. This corresponds to the frequency FlM or P2M, which is chosen to represent the marking condition. Conversely, when send contacts ZfPZ are open, corresponding to a spacing signal, shunting inductor TZB is removed from the tank circuit, as previously described, whereby the frequency is decreased in the basic frequency band. This corresponds to the spacing frequency F or F25.

The collector of transistor QZB is connected to negative battery by way of resistor RtBB, make contacts M-12 or CON-12 in parallel, and resistor R7B. Relays M and CON are closed at appropriate times in the control sequence to start the oscillator as described hereinafter. The feedback path for the oscillator consists of resistor RNB. Varistor RVllB across terminals 2 and i of inductor TlB, which Varistor may suitably comprise two reversibly poled diodes in parallel, stabilizes the amplitude of the oscillator so that linear operation may be obtained.

The output of the oscillator is taken from the collector of transistor QEB and proceeds through an appropriate filter to the amplifier transistorQlB. Two filters are provided comprising a bandpass filter for the F1 band, generally indicated by block lili, and a bandpass filter for the F2 band, generally indicated by block 102. When the data set is in the originating mode, relay OR is operated and relay AN is released, as described hereinafter, the oscillator is tuned to the F1 band, the F1 filter 101 is put in the transmitting branch, the F2 filter 102 is in the receiving branch. The output of the oscillator in this case is connected from the collector of transistor QZB through the make contacts of contacts Oli-7, F1 filter lfi, send level adjustment pad N3, make contacts of contacts OR-li, send level adustment pad 104, and capacitor CZB to the base of transistor Q1B.

Conversely, when the data set is in the terminating mode, relay AN is operated and relay OR is released, the oscillator is tuned to the F2 band, F2 filter 102 is intro- 8 duced in the transmitting branch, and F1 filter lill is in the receiving branch. The output of the oscillator in this case is connected from the collector of transistor QZB through the break contacts of contacts OR-7, F2 filter 102, the break contacts of contacts OR-ll, pad 104, and capacitor CZB to the base of transistor QlB.

As described above, the appropriate filter and level adjustment pads for each mode of operation are inserted in the transmitting branch. Level adjustment pads provide adjustment of the send level as is well known in the art. It will be recalied that the F2 band has a higher frequency than the F1 band. It may, therefore, be desirable to transmit the F2 band at a higher level than the F1 band. This result is obtained by placing pads 103 and 164 in the series with the transmitting branch when transmitting in the F1 band.

Transistor QlB is la power stage which provides sufficient signal power to drive the telephone line and isolates the line from the bandpass filters. Transistor QlB is a standard common emitter voltage amplifier with its collector output taken between resistor RTB and resistor RZB and fed to the hybrid, generaliy indicated by block N5.

Hybrid coil 165, which i-s partially shown in FTG. 1, includes hybrid coil TZC and 4hybrid coil TC. The output of amplifier transistor QiB is placed across terminals 6 and 7 of hybrid ycoil TlC. As is Well known in the art, the resultant signal is applied across rterminais l and 4 of hybrid coil TiC and isolated fro-m hybrid coil TZC. Terminal l of transformer TlC is connected to the ring terminal lof the telephone line tand terminal i of transformer TiC is connected to the tip of the telephone line by way yof the make contacts of contacts AN-lZ and `the break contacts of contacts Oil-l2, when the data set is in the terminating mode, for by Way of filter M6, lead T108 and dial pulse contacts 11437 in parallel, 'and the make centacts =of contacts OR-12 when the data set is in the originating mode. Thus, the output across terminals 4 `and 1 of transformer TiC is applied across the tip `and ring terminals of the telephone line.

Limiter As previously described, the tip and ring terminals of the telephone -line yare `connected .to terminals 4 Zand i, respectively, of transformer TEC. Accordingly, .sign-als entening from the telephone line are `app-lied across trainsformer TlC 'of hybrid 105'. As is Well known in the art, these signals tare `applied across terminals 6 `and Iii of transformer TEC and isolated lfrom the TlC coil across terminals 6 and 7. The output of transformer TZC is applied to the base 'of transistor stage QtD, FIG. 3, of the limiter by way `of lead 109 and resistor RSD.

Transistor QLD is .a buffer amplifier to isolate the line from the receiving bamdpass yfilter and to provide a means for adjusting the gain of the limiter. The input impedance of transistor QED is adjusted to match the receiving branch impedance of the hybrid coil Iby choosing appnoprilate values for resistors Rail), R51) and RcD. Potentiometer RSD in the emitter circuit provides a gain adjustment. The listen-only hand set ifi-'9, FG. l, is connected lin series with the collector ground supply of transistor QlD via lead 3&1 and resistor RiD to permit audible monitoring of supervisory signals such las busy back tones and audible ringing when the call is vbeing set up. After the call has been set up, the use of ya hand set is .no ionger required and the closure of relay contact CON-9 at that time short-circuits the hand set through ycapacitor Cil). The `oif-norrrial dial contact M2 also short-circuits the hand set to prevent ldial puises from disturbing the listener while dialing.

The output of the collector of transistor QED is connected through lead Stil and lead lf2 to the OR relay switching arrangement so that the appropriate fiiter is switched into the receiving circuit. If the station'is in the originating mode, relay OR is operated and the dis- 'ariane criminator is tuned to the F2 band as described hereinafter. Thus, with relay OR operated, the output signals from transistor Qll) are afplied via leads 3M and lZ, the make ycontacts of contacts OR-l, F2 filter 102, the make contacts of contacts Oil-4, lead M3, capacitor C3l), FlG. 3, and resistor R91) to the base of transistor QZl).

the station is a terminating station, rel-ay Ol?. is released, the discriminator is tuned to the F1 band, and the output signals from transistor QlD nare applied by way of the break contacts of contacts GR-ll, F1 filter lill, the break contacts of contacts Oli-4l, lead H3, capacitor CSD, and resistor RQD to the base of transistor QED.

Transistor Q2l), which comprises the first limiting stage, limits by virtue of the A.C. feedback connection from the collector to the base through capacitor Cdl) and varistor RVlD. The appropriate bias for the base of transistor 2D is obtained from the junction of resistors lii'l) and RSD which are connected in series with resistors RMD `and R141) between ground and positive battery. When the peak to peak output at the collector of transistor (22D exceeds the threshold of varistor RVH), the varistor conducts causing negative feedback to reduce the gain of the stage. Thus, my signal which exceeds a predetermined `amplitude will be limited by the action of this eedbact circuit. rilse output from the coll lector of transistor (22D is fed through capacitor CoD and resistor R131) to the base of transistor QED which is the second limiter stage.

The operation and arrangement of transistor Q31) is substantially identical to that of transistor QED. rihe feedback path of transistor Q31) is shown comprising capacitor Cl) and. varistor RJiD, in series. lt is understood, of course, that two or more var-istors may be provided in series to reduce the feedback and thereby provide less limiting as determined by the desired output level.

The particular stages which limit depend upon the input signal level. For high input levels, both stages limit to some extent. For `low input levels, only the final stage performs the limiting function. Accordingly, the trequency shift signal provided to the output of transistor Q39 -is substantially Iuniform in amplitude and rectangular in form due to the limiting action of transistor stages Q21) and Q3l).

'Ehe collector output of transistor QED is l).-C. coupled to the base of driver transistor Qdi). The collector of transistor Qdi) drives the discriminator and the emitter of transistor QlD drives the space-hold detector circuit as described below.

The space-hold detector consists of resistors Rltl), Rll), lZSD, RZSD and R241), thermistor RTlD, varistors RVl) `and RVoD, diode Cll), and capacitors CHD, CMD andl ClSD all shown in FIG. 3. The function of the space-hold detector is to make a judgement as to whether or not a signal is behng received from the line at a level above a minimum lacceptable limit. lf a signal above this level is being received, the space-hold detector allows the incoming signals to pass freely through the discriminator. lf the incoming level from the line is below this limit, the space-hold detector is designed to block transmission through .the discriminator from the line `and hold the discriminator output in the spacing condition.

The output of the space-hold detector is obtained at the junction of diode CRlD and capacitors ClZD and Cl3l) and applied by way of lead 392 and OR gate diode CRllA, lG. 4, to the input of transistor stage QZA in the discriminator. In the absence of received signals, the voltage of space-hold control lead 362 is negative due to a negative bias voltage applied by way of resistors R19D, Rll), -R2D and RZSD. This negative voltage causes diode CREA to conduct, thereby holding transistor QZA in the On condition. `During the reception or" signals, the limited wave provided at the emitter of transistor Q4l) is applied to capacitor CMD. The righthand plate of capacitor CHD, as shown in FIG. 3, is

connected to gro-und by way of resistor lZflD and varisfl tor RVl). Thus, a wave having an amplitude related to the amount that the received signal wave is limited is applied to diode CRll) which is connected to the junction of capacitor ClilD and resistor RMD. Diode CRlD in turn passes `the positive portions of the wave to oppose the negative bias on lead Stil?. When the level is high enough so that the positive output voltage from diode CllD overcomes the negative voltage on spacehold control lead 3M, diode CllllA no longer conducts, isolating transistor QZA from the space-hold detector.

The function of thermistor l'lD and varistors RVSD and RVdD, together with resistors Ridi) and R251), is to cause a variation with temperature of the negative bias component of the space-hold output voltage, since tre bias component is obtained from the voltage divider circuits in which these resistors and temperature sensitive elements are situated. This provides a compensation for the variation with temperature of the positive voltage which is derived by diode CRll) from part of the limiter output voltage.

In the local modo, which will be described later, it is necessary to disable the space-hold circuit. For this purpose, the negative voltage bias source is short-circuited to ground through lead Silit and the break contacts of contacts Gil-itl and AN-, 4. When the data set is switched to the originating or terminating mode, relay OR or relay AN operates and the negative voltage charge rapidly builds up on lead 3.532 to render the space-hold circuit effective. QFor this reason, the value of capacitor (212D, whicn is connected across lead 3%2 and ground, is relatively small. When the connecting sequence is completed, relay CON operates, as described hereinafter, and the capacity is increased by extending ground to capaictor `Clll) by way of make contacts CGN-3 thereby connecting capacitor (233D in parallel with capacitor Cll) to minimize the fluctuations of the space-hold voltage due to amplitude variations of the limiter output. When the received signal is low and near the operating threshold circuit, such fluctuations in the space-hold output voltage umight produce an undesired amplitude modulation of the signals passing through diode CRllA to transistor stage QZA.

The discriminator, FIG. 4, which consists of four parallel tuned circuits in series composed of inductors TlA, T2A, TSA and TiA and capacitors C1A, CEA, CEA and CdA, is driven by the collector current of transistor Qill) by way of lead 3dS. The secondaries of the inductors are selected in pairs by means of relay contacts Md and M-Z which short-circuit the undesired secondaries. With a shortcircuited secondary in one of the tuned circuits, the primary circuit resonance Afor that circuit is shifted sufficiently high in frequency so as to have no influence on the resonance of the remaining coils.

lf a station is in the terminating mode, relay M is operated as described hereinafter, and the secondaries of inductors T2A and TdA are short-circuited. The activo elements of the discriminator in this case are inductor TlA and capacitor ClA in shunt thereto and inductor TSA and capacitor CdA in shunt thereto. With these tuned circuits active, `the discriminator has the normal S-shaped curve rendering the discriminator effective in the F1 frequency band. ln addition, indu-eter TlA together with the capacitor ClA is tuned to excite the diode bridge consisting of diodes CRlA through `GBAA in response to the space frequency and the inductor TSA together with capacitor CSA is tuned to excite the diode bridge consisting of diodes CREA through `CESA in irespense to the m rking frequency. lf the station is in the original mode, relay M is released and the secondaries of inductors TlA and TSA are short-circuited, rendering active the tuned circuits comprising inductor T2A together with capacitor CZA and inductor TeA together with capacitor CtA. In this case, the discriminator is tuned to the F2 band as is appropriate for an originating station.

The tuning of the di-scriminator network is such that the peaks of the resonances are slightly higher and lower than the mark and space frequencies. This produces a linear frequency-to-amplitude conversion. The amplitude output is then detected by the diode bridges and developed across load resistors REA and RSA. Capacitors CSA and `CoA are smoothing capacitors to lter out the carrier ripple. Since the `diode bridges are oppositcly poled, a mark frequency coming into the discriminator will develop a positive voltage across the output load and a spacing frequency will develop a negative polarity across the output load. These signals are fed through the low pass lilter consisting of inductor L1A and capacitor `C7`A to the base of transistor amplifier stage QlA. Provision is made to adjust the position of the discriminator S curve by means of potentiometer RffiA.

Transistor QlA is an emitter follower with load resistor R6A. The application of a negative spacing signal to the base of transistor QllA increases the emitter-tocollector current providing a negative signal at the junction of load resistor RGA and resistor RQA. Conversely, a positive ymarking signal applied to the base of transistor QlA drives the voltage at the junction of resistors IR6A and RQA in a positive direction due to the application of positive battery by way of resistor RMA.

The output taken from the emitter of transistor Q1A drives the base of transistor amplifier stage QZA through resistor RQA and OR `gate diode `CRt/A. The space-hold clamp described labove connects to the base input of transistor QZA through another `OR gate diode CRllA. The collector output of the break timer transistor stage QKlB connects to the base of transistor QZA over lead 114, resistor RlSA and through a third OR gate diode CR14A. This latter circuit causes the locally-sent signals to be repeated by transistor :amplifier QZA and subsequent stages to the teletypewriter select magnet, as described hereinafter, providing local copy.

In the quiescent condition, the teletypewriter is sending a continuous marking signal and the demodulator is receiving a marking frequency. Under this condition, assuming the received signal is at a desired amplitude whereby the space-hold detector overcomes the negative bias on lead 392, none of diodes CR9A, CRllA or CRMA is applying a negative potential to the base of transistor QZA, whereby the transistor is in the nonconductive condition. ln the nonconductive condition, negative battery is applied to the collector of transitor QZA by way of resistor RlSA. When a spacing signal is being sent by the teletypewriter or being received by the demodulator, or the received signal level is below the threshold level, a negative potential is applied to the base of transistor QZA as previously described, and the transistor is in the conductive condition whereby the collector is substantially at ground potential.

During the application of a negative spacing signal to the base of transistor QZA, leakage current is drawn through diode CREA from the space-hold detector capacitors CILZD and CHD via lead 362. In the event that the signal level just exceeds the permissable level, the leakage current may re-establish the negative bias on lead 3192. At the termination of the spacing signal an interval is required by the space-hold detector to overcome the negative bias whereby distortion of the signals results. To compensate for the leakage current through diode CRllA, a clamp comprising diode CRlSA and resistor RMA is connected between the collector of transistor QZA and lead 392. When a spacing signal is received, the collector of transistor QZA is at ground potential, removing the back bias on diode CREA and providing current to diode CRllA to compensate for the leakage current.

The collector output of transistor QZA is connected through Zener diode CRlZA to relay contacts CYl. Relay CY is unoperated prior to the connection of the teletypewriter to the output of the demodulator. With relay CY released, the output of transistor QZA is fed through the break contacts of contacts CY-L leads till and 592, and resistors RWE and RlSE, FIG. 3, to positive battery controlling the signal applied to the base of timer input transistor QSEl at the junction of resistors REQE and RlSE. With this connection, a positive potential is applied to the base of transistor QSE via resistors RlSE and RME, leads itil and 492., and Zener diode CRlZA when the collector of transistor QEA is at the spacing ground condition. When the collector of transistor QZA goes to the marking negative condition, a negative potential is applied to the base of transistor QSE by way of Zener diode CRZA conducting in the reverse direction and leads itil and 492 and resistors RNE and RlSE.

rl`he timer circuit subsequently operates relays CON and CY, as described hereinafter, connecting the collector output of transistor QZA to positive battery by way of Zener diode CRlZA, the make contacts of contacts CY-l and resistor RlZA, controlling the signal applied to the base of transistor QSA via resistor RHA. Accordingly, with this connection, a marking signal produces a negative voltage and a spacing signal produces a positive voltage at the base of transistor QSA.

The application of marking negative voltage at the base of transistor QSA turns it On causing current to flow from ground through the emitter-to-collector path of transistor QSA, lead 403, and resistors RIG and RZG in the keyer circuit, FIG. 2, to negative battery. This current ilowing into the keyer circuit results in lcurrent through select magnet 205 of teletypewriter Zilli as described hereinafter.

Transistor Q3A is cut off when a positive spacing signal is applied to its base, driving its collector negative clue to battery applied by way of resistor RMA. Since relay CON is operated, this negative voltage is fed through resistor R19A, make contacts CON-2, lead 402, and resistor R19E to timer input transistor QSE. Prior to the operation of relays CON and CY, transistor QZA applies negative signals corresponding to marking signals and positive signals corresponding to spacing signals to the timer. Following the operation of relays CON and CY, the timer is connected to transistor Q3A which provides a logical inversion. At this point, the application of a negative voltage by transistor stage QSA to the timer corresponds to a spacing condition, and positive, ground voltage corresponds to a marking condition.

Keyer The keyer, or electronic select magnet driver, comprising transistors Q1G through QlG, FIG. 2, amplies thediscrirninator output suiciently to drive select magnet 295 of teletypewriter 201. The ydiscriminator output transistor stage QSA feeds current in response to marking signals through input resistors RlG and RZG of the keyer as previously described. Accordingly, the junction of these resistors has a negative potential determined by the negative battery applied via resistor RZG during a spacing signal and a potential relatively positive thereto during a mark-ing signal. These voltages are fed through resistor R13G to the base of transistor QlG.

Transistors QlG, QZG and Q4G :form a trigger input 'circuit which has two stable states. This circuit triggers in one direction when the input voltage rises above some threshold value and goes back to its original state when the voltage falls below this value.

Transistors QSG and QLEG constitute switches that control the application of current through select magnet 20S in accordance with the `condition of the bistable input trigger circuit. Transistor QiG and diode CRlG provide a controlled discharge circuit for the select magnet in- 13 ductance when the input signal goes spacing and the current through select magnet S is removed as described below. This discharge circuit thus prevents the generation of high induced voltages across the inductance of the select magnet winding.

When the input to the keyer is spacing, all four of transistors QlG through Q4G are in -a nonconductive condition. The base of transistor QZG is connected to the junction of resistors R9G and R6G, which resistors are connected in series with resistor RSG between negative and positive battery. Accordingly, the base of transistor QZG is positive relative to the potential on the base of transistor QlG during spacing signals. Since the emitter of transistor QlG is `connected to the emitter of transistor QZG, a back bias is developed across the two emitters to base junctions in series and neither transistor conducts.

The collector of transistor QlG is connected to positive battery by way of resistor RSG and directly cormected to the base of transistor Q4G. The emitter of transistor QdG is connected by way of resistor RSG to the junction of resistors RG and RQG, and is connected to positive battery by Way of resistors R12G and RUG. This renders the base of transistor QlG positive relative to the potential on the emitter so that transistor QfG is back biased and non/conducting.

The base of transistor QBG is connected to the junction of resistors Rl-lG and RZG, which resistors are connected in series with resistor RSG between positive battery and `the junction of resistors RSG and l`9G. The emitter of transistor QSG is connected to the junction ot voltage divider resistors REG and RG. The values of resistors RSG, EGG, RSG, RltilG and RZG are arranged to provide `a potential at the base of transistor QSG relative positive to the emitter potential whereby transistor QBG is back biased yand therefore noncond-ucting.

When -a marking signal is applied by the discriminator output to the kcyer, the voltage on the base of transistor QlG is driven in a positive direction. Since this base voltage is more positive than the base voltage of transistor QZG, both transistor QlG and transistor QZG become forward biased and conduct. When transistor QlG conducts, its collector voltage is driven in :a negative direction applying negative potential to the base of transistor Q-G. The application of the negative potential to the base of transistor Q4G causes transistor QiG to become `forward biased. With transistor Qfi-G conductive, its emitter potential is rendered negative and this negative potential is applied by Iway of resistor RG to the base of transistor QSG. ln addition, the negative emitter potential of transistor Q4G is applied by way of resistor RSG to the base ot' transistor QZG to provide positive feedback by increasing the forward bias on transisters QZG and QlG.

The application of ythe negative potential to the base of transistor Q3G turns the transistor On. This results in current from positive battery by `way of resistor RSG, the emittente-collector path of transistor QSG, the winding of select magnet ZS, the emittente-collector path of transistor QftG, and resistor RltG to negative battery. Thus, the application of a marking signal to the keying circuit provides `marking current to select magnet 2%.

When the input signal to the keyer goes to a spacing condition, the base oi transistor QlG is driven to the initial negative condition causing transistors QlG and QZG to become back biased. When transistor QlG turns @if the resultant positive collector potential of transistor QlG is applied to transistor Q-iG, turning Gti the latter transistor. When transistor Q-lG turns Off, the positive potential on the base of transistor QSG is restored and transistor QEG is again rendered nonconductive. rthus, the applic-ation of a spacing signal to the keyer circuit removes the marking current applied through the winding of select magnet 255.

Timer The timer circuit contains transistor stages (E,

Qii-E, QSE, QZE and QlE, FIG. 3. The timer per-forms several diierent `functions depending upon the mode of operation of the data set. It the data set is in the answer mode of operation, immediately after the answering station has gone oit-hook the timing circuit will time for a period of about one second. During this interval, indicated hereinafter as a guard interval, the terminating or answering station does not transmit any signal to- `ward `the originating station. Following the `guard interval, the timer is switched to monitor -for continuous marking frequency at the output of the transistor stage QA in the discriminator and will time-out after approximately one second of such marking signal. Once the `markin g signal time-out has occurred the input of the timing circuit is transferred to a subsequent stage in the discriminator. The timer then monitors for the reception of a spacing signal one second or more in duration. When the timer detects the one second spacing signal it will time-out and initiate the disconnect sequence as described hereinafter.

lf the station is an originating station, the timing circuit will not time for the guard interval. The timer will first look `for a marking signal lfrom the terminating station and time-out after approximately one second of such marking signal. The timer will then transfer to a monitor space condition waiting to detect a long spacing signal calling for a disconnect.

At the originating station when the timing circut is ymonitoring for the reception of the `marking signal, the output or transistor QZA in t1 e discriminator is connected t trough Zener diode CRlZA, the break contacts of contacts CYlB, leads tll and 462, and resistor RiE to the base of transistor QSE as previously described. The reception of a spacing signal causes transistor QZA to conduct connecting ground at the vemitter through to the collector and over the path traced above to the base of transistor QSE land then through resistor Rig-E to positive battery so that the base voltage of transistor QSE is rendered positive. The emitter or' transistor QSE is biased slightly positive relative to ground by the voltage divider comprising resistors lldE and R'E. Thus, transistor QSE is rendered nonconducti've during the reception of a spacing signal or, due to the space-hold detector circuit, in the absence of a received signal.

V'Jith transistor QSE Ori a negative potential is applied to the base of transistor QAPE by vvay of resistor RlSE whereby the positive potential on the emitter of transistor QdE applied by way ot resistor RlSE makes transistor QliE conductive. With transistor Q-E conductive, the emittente-collector current is applied by Way of resistor RME to negative battery rendering the emitter potential negative. This negative potential is Vapplied via resistor REE and diode CREE to the upper plate or" timing capacitor CSE as shown in PlG. 3.

ln the originating station, relay AN is released whereby timing capacitor CSE is connected to the base of ltransistor QSE through break contacts ANS and the break contacts of contacts 8 7. The emitter of transistor QSE, it is noted, is connected to ground by way of diode CllE. The lower terminal of capacitor CEE is connected through resistor RSE and break contacts CGN- 7 to ground. Accordingly, as long as transistor QSE is Oi, a negative voltage is maintained across capacitor CSE and the oase of transistor QEE is `held negative with respect to its emitter, maintaining transistor Q3E nonconductive.

When a marking signal is received from the terminating station, under this situation, transistor QZA in the discriminator turns Ott` thereby applying negative battery through resistor RISA, Zener diode CREA, the break contacts of contacts CY-l, leads tdi and 462, and resistor R195 to the base of transistor QSE as previously described. This negative-going signal will cause transistor QSE to conduct.

When transistor QSE conducts, the potential at the l collector will be drivenpositive. This causes the base of transistor QE to go positive and the voltage on the emitter of transistor QE Will follow that of the base causing diode CRZE to cease conducting since its left-hand terminal, as shown in FIG. 3, is at the negative potential of the upper plate of capacitor CEE. When diode CRZE ceases to conduct, a charging path is established for capacitor CSE toward positive battery through resistor RNE and through resistor R'iE by way of the break contacts of contacts S-7 and break contacts AN-5. Capacitor CSE will charge slowly until the base of transistor QE goes slightly positive rendering the transistor conductive. In the event that a spacing signal is received While capacitor CSE is charging toward positive battery, transistor QSE is again turned Oil, the emitter of transistor Q4E is again rendered negative via resistor RESE, and the negative charge is reapplied to capacitor CSE by Way of diode CRZE.

Assuming now that the received marking signal is maintained until capacitor CBE goes slightly positive, and transistor QSE conducts, the voltage at the collector will drop toward ground. The collector of transistor QSE is connected to the base of transistor QZE by way of resistor RSE which forms a voltage divider with resistor R4E between the collector of transistor QSE and negative battery. Accordingly, the negative-going potential change of the collector of transistor QSE renders the base of transistor QZE negative in potential. Since the emitter 'of transistor QZE is biased to a potential just above ground by the voltage divider comprising resistors RIE "and RZE, transistor QZE conducts when its base voltage '1s rendered negative and will operate as an emitter follower. Since the base of transistor QIE is connected to the emitter of transistor QZE and the emitter of transistor QlE is normally grounded through the break contacts of test key contacts 305, transistor QlE will conduct when the base voltage of transistor QZE is rendered negative and the emitter of transistor QZE, following its base Voltage, drives the base of transistor QlE negative.

At the originating station, relay OR is operated as described hereinafter. The collector of transistor QlE is thus connected through lead 306, the make contacts of contacts OR-S, FIG. 4, break contacts S-l, the break contacts of contacts CON-6, and the winding of relay CON to negative battery. Thus, at an originating station, the reception of a continuous mark of about one second will turn on transistor QlE and the resultant emitter-to-collector current of transistor QlE operates relay CON.

fThe operation of relay CON does two things which effect the timer. The detailed function resulting from the yoperation of the relay CON will be described hereinafter. Relay CON operated operates relay CY shifting the timer input from the collector of transistor QZA to the collector of transistor Q3A as previously described. Because of the signal inversion in going through the additional transistor stage Q3A, a negative-going signal at the collector of transistor Q3A now corresponds logically to a spacing signal rather than a marking signal. Therefore, when connected to the output of transistor stage Q3A, the timer will monitor for spacing signals and will be reset by the reception of marking signals.

As mentioned above, the operation of relay CON operates relay CY. Make contacts CY-S in conjunction with the make contacts of contacts CON-1 connect timer capacitor CIE into the timing circuit. The previous timing capacitor CSE is disconnected by break contacts CON-7. vSpacing signals will then be monitored by the timer and any spacing signals which occur will turn On transistor yQSE whereby capacitor CllE charges toward positive volt- .age by way of the make contacts of contacts CON-1 and resistor R7E, make contacts CY-S or the break contacts of contacts S-7 in shunt thereto, break contacts AN-S and resistor RNE. The time constant in this case is adjusted so that a continuous spacing signal of about one ISecond Will result in biasing the base of transistor QSE positive causing transistor QSE to conduct. This will turn on transistors QZE and Q1E. The collector output of transistor QlE is now connected through lead 306, the make contacts of contacts OR-S, and the make contacts of contacts CYLS to the Winding of relay S. Accordingly, relay S operates indicating that a spacing signal of one second or more has been received. This initiates the disconnect sequence which will be described more fully below.

The sequential circuit action of the timer at the originating station is substantially the same as the sequence at a terminating station except that there is an additional timing interval of about one second which is introduced prior to the transmission of the marking frequency P2M toward the originating station. This extra interval is a period of no signal transmission to the line and is called tl e guard interval. It is necessary to hold the application of signals to prevent interference with the propagation of the oli-hook signal which originates certain operations at the central oce.

The terminating mode as distinguished by the operation of relay AN is described hereinafter. The operation of relay AN opens break contacts AN-S interrupting the previously described monitoring mark connection between the base of transistor QSE and timing capacitor C3E. In addition, relay AN operated closes the make contacts of contacts AN-6 connecting the base of transistor QSE to the guard timing capacitor CZE by Way of the break contacts of contacts S7, the break contacts of contacts M-4, and make contacts of contacts AN-6. Prior to the operation of relay AN, timing capacitor CZE charges to negative battery through resistor R5 and the break contacts of contacts AN-6. After relay AN operates, this negative charge is applied to the base of transistor QSE maintaining the transistor nonconductive. Capacitor CZE, however, now charges toward positive battery through the make contacts of contacts AN-G, the break contacts of contacts M-4, the break contacts of contacts S-7, and resistor R7E until the base of transistor QSE becomes positive. The time constant of capacitor CZE and resistor R7E is such that this will occur in about one second after relay AN operates. When transistor Q3E conducts, transistor stages QSE and QlE turn On connecting ground on the emitter of transistor Q1E through the emitter-to-collector path of the transistor and then through lead 306, the break contacts of contacts OR-S, make contacts AN-9 and the break contacts of contacts M-S to the Winding of relay M. Operation of relay M turns on the oscillator in the modulator as previously described causing the marking frequency P2M to be transmitted to the originating station. The -opening of the break contacts of contacts M-4 disconnects the base of transistor QSE from the guard timing capacitor CZE and the closure of the make contacts of contacts M-4 connects the base of transistor Q3E to timing capacitor C3E.

Conditions now are the same as in an originating station wherein the timer circuit is monitoring for the reception of a continuous marking signal. The timer will proceed to operate relay CON at the expiration of the one second marking interval and the further operation of the timing circuit is identical with the operation previously described for the originating station.

Answer-Back Timing Circuit At an unattended station arranged for automatic answering, an additional timing circuit is provided by transistor stages QlF and QZF, FIG. 2. This timing circuit disconnects the station from the loop if the station fails to get a marking signal from the originating station within a predetermined interval of time after the terminating station concludes the guard interval by sending P2M. This condition would occur if a telephone subscriber dialed a data station by mistake.

The base of transistor QZF is supplied positive battery through resistor RF and break contacts M-3. The emitamarre ter of transistor QZF is connected to the junction of resistors RIF and RSF which resistors are connected across positive battery and ground. This renders the base of transistor Q2F positive relative to the potential at the emitter and transistor QZF is normally nonconductive. With transistor QZF nonconductive, the emitter of transistor QZF connected to the base of transistor QlF, and the emitter of transistor QlF connected to ground, transistor QlF is turned Off.

When a call is answered by an unattended station, the timer circuit generates the guard interval as previously described. At the end of this interval, relay M operates opening break contacts M-3 and disconnecting positive battery from the base of transistor QZF and from capacitor ClF, which is connected to the base of transistor Q2F. Capacitor CIF thus begins to charge slowly toward negative battery through resistor RAF.

If the call is from another data set, a marking signal is received from the originating station causing the operation of relay CON as previously described. The consequent closure of make contacts CON-4 reapplies positive battery to the base of transistor QZF and to capacitor CEF. Accordingly, the timing circuit will be recycled before it times out.

In the event that the call is from a telephone subscriber the marking signal is not received and capacitor ClF continues to charge toward negative battery until the potential on the base of transistor QZF is rendered negative. The application of the negative potential to the base of transistor QZF turns the transistor On driving its emitter potential negative. The application of the negative emitter potential of transistor QZF to the base of transistor QlF turns this latter transistor On. A current path is then available to operate relay S from ground to the emitter of transistor QF through the emitter-to-collector path of transistor QlF, lead 297, and the break contacts of contacts S-6 to the winding of relay S. The operation of relay S at this time causes the release of relay AN as described hereinafter, restoring the station to the on-hook condition.

Answer-Back Generator For those stations that are arranged for automatic answering, an answer-back generator is provided to inform the originating station that the terminating station has answered and is ready to receive a message. This answerback generator is arranged when activated to cause the transmission of the teletypewriter character v The answer-back generator consists of a one-shot multivibrator. Transistors QMBF and QMF, FIG. 2, are the active elements of this circuit. The base of transistor Q11F is connected to negative battery via resistor RllF and is therefore normally conducting. The base of transistor QllF is connected to the junction of resistors R13F and RlSF which resistors are connected in series between positive battery and the collector of transistor QMF. This applies a positive potential to the base of transistor QltlF whereby the transistor is normally Oft. The input circuit includes negative battery connected through resistor RSF, make contacts M-ll, make contacts CON-11, and resistor RM1-T which is connected to the lower plate of capacitor CMF as shown in FIG. 2. The upper plate of capacitor CHF is connected to the base of transistor QltlF.

Transistor QlltlF is caused to conduct by a negativegoing pulse -through capacitor CHF. Such a pulse is generated in a terminating station when relay CGN operates at the termination of the connect sequence. Since, as previously described, relay M operates at the terminating station during the connect sequence, the operation of relay CON provides a negative transition to the lower plate of capacitor CMF by way of make contacts CON-11.

The application of the negative-going voltage transition to capacitor CMF provides a negative pulse to the base of transistor QilllF turning the transistor On. The resultant positive-going change in the collector voltage of transistor QitlF is applied through capacitor C10F to the base of transistor QlllF turning the latter transistor Ofi. Transistor QllF remains cut olf until capacitor CltlF charges up again toward negative battery via resistor RHF thereby reapplying a negative potential to the base of transistor QllllF. At this time, transistor QllF will go 0n again and the resultant positive-going change in its collector voltage is applied through capacitor C13F to the base of transistor QlltlF turning this transistor Off.

The output of the generator is taken from lead 208 which is connected to the collector of transistor QlF. During the normal condition when transistor QltlF is Oil, negative voltage is applied to lead 29S by way of resistor ENF. This voltage is blocked by diode CRltF, FIG. l, which is connected to the base input of the break timer transistor QtB via capacitor C63. It may be recalled that break timer transistor QiB is normally driven by sending contacts 203 and in turn feeds keyer transistor stage QSE.

When transistor QltiF turns On, a positive-going potential is provided at its collector. This positive-going transition is transmitted through lead 208, diode CRltF, and capacitor C63 to the base of break timer transistor QfsB, turning it Gif, and thereby causing a spacing signal 'to be transmitted -to the line. When the monostable ipflop circuit comprising transistors QltlF and Q'llF relaxes to its normal condition, the negative voltage condition o lead 20@ is restored, thus back biasing diode illdF. The reapplication of negative battery to capacitor C6B by way of resistors R133 and RMB restores transistor QrB to the marking condition as previously described. Accordingly, a spacing signal is transmitted having a duration equal to the interval that the monostable multivibato-r is in the unstable state. The. duration of this interval is arranged to provide a signal simulating the teletypewriter character V.

Originating a Call When it is desired to originate a call, the originate key is depressed closing-the normally open contacts 464 of the originate key, FlG. 4. Operation of this key causes the operation of relay GR over a path through the break contacts of contacts S-S, the break contact-s of contacts AN-3, the originate key contacts 4MM, the winding of relay OR, and the break contacts of contacts S-9 to negative battery. Relay OR locks up through the make contacts of contacts Olii-2 and the break contacts of contacts Abi-4 to ground and removes blinding ground applied by way of the break contacts of contacts AN-4, the break contacts of contacts yOil-2, and lead llo to the base of transistor QlA of the discriminator.

Operation of relay Oi?` places the data set off-hook by connecting a direct current path from the tip to the ring terminal of the 'telephone line via the make contacts of contacts Oil-l2, dial pulse contact 107, tilter lilo, -tcrminal 4 to terminal 8 of hybrid coil TC, terminal l to terminal 3 of hybrid coil TZC, and terminal 9 to terminal ll of hybrid coil TlC. ln addition, relay `OR operated lights originate lamp 405, FlG. 4 over a path lfrom ground through the break contacts of contacts S-, the make contacts of contacts OR-, the filament of lamp 465 and resistor R-d to positive battery and opens a possible operating path for relay AN by way of break contacts of contacts OR-S. Operation of relay OR switches the F1 ilter del into the sending branch by way of make contacts of contacts (ER-'7, switches the F2` filter -into the receiving branch by way of make contacts of contacts OR-l and contacts Gli-4, and inserts pad lil-3 in the transmitting branch by way of the make contacts of contacts OR-l as previously described. By opening the break contacts of contacts (3R-9, FIG. 4, relay OR removes `a negative disabling clamp provided by yway of resistor R-Z, the break contacts of contacts AN-lll, the break contacts of contacts Oil-9, lead 3M, and resistor RQE, FG. 3, to the junction of diode CRZE and capacitor CSE to permit the timer to monitor for a marking signal received from the telephone line. By closing the make contacts of contacts OR-9, a negative marking signal is applied to the base of transistor `QaA blinding the transistor to signals. This function is only utilized upon disconnect-ing the stations, however, -as described hereinafter. Also, the operation of relay OR opens the previously described disabling path for the space-hold detector from ground through the break contacts of contacts AN-'7, the break contacts of contacts OR-ltl, and lead 304. Finally, relay OR operated connects the output of the timer applied to lead 3% to the winding of relay CON by way of the make contacts of contacts (3R-3, the break contacts of contacts S4, and the break contacts of contacts CON-6.

After the operation of relay OR, the o-hook signal into the central ltelephone otlice results in dial tone applied across the telephone line. This dial tone, as previously described, is applied across terminals 6 and 8 of hybrid coil TZC and then by way of lead 1639 to transistor amplifier QllD which, in turn, supplies the dial tone by way of lead Stili to listen-only hand set i199; The data set attendant, hearing the dial tone, now dials, momentarily opening dial pulse contacts ltl thereby opening and closing the previously described tip and ring connection.

ln response -to the dial pulses the calling party is connected through the telephone network and ringing signal is .transmitted to the called station in the conventional manner. The calling attendant will hear the audible ringing tone in listen-only hand set M9; if some diiiculty is encountered in setting up the connection, the attendant hears the conventional supervisory tones a normal telephone subscriber would hear. As described hereinafter, the terminating station, in response to the call, provides an initial guard interval followed by the transmission of the P2M marking signal.

The originating station receives the P2M marking signal from the termin-ating station and this frequency is detected lby the discriminator whereupon the timer proceeds to monitor the discriminator output for a one second mark by way of the break contacts of contacts CY-l and leads 4642i land 402, as previously described. At the expiration of the one `second marking interval, transistor QiE conducts, as previously described, whereby current is provided from ground by way of test key contacts 305, the emitter-to-collector path of transistor Qll, lead 3%, the make contacts yof contacts OR-S, the break contacts of contacts S-l, and the break contacts of contacts CON-6 to the winding of relay CON. Relay CON locks to ground by way of the make contacts of contacts CON-6, the make contacts of contacts OR-w, and the break contacts of contacts AN-7.

Relay CON operated extends ground by way of make contacts CON-9, FlG. l, to capacitor ClD, thereby shortcircuiting the received tones previously applied to listenonly hand set 169. The closure of make contacts CON- l?, extends negative battery to the collector of oscillator transistor QZB, as previously described, thereby turning On the oscillator. Since relay AN is released and send contacts 292 are closed, the oscillator transmits frequency FIM.

Relay CON operated also completes 'an operating path for relay CY by way of make contacts CON-5, and break contacts S-S. In addition, relay CON operated disconnects negative battery from timing capacitor 01E by way of the break contacts of contacts CON-ll and connects capacitor CIE to the base of transistor (23E by way of the make contacts of contacts CON-lt, preparing the timer to monitor for a one second spacing signal as previously described. Finally, relay CON operated extends ground by way of make contacts CON-3 to the space-hold detector capacitor C131), enabling the capacitor, as previously described.

Relay CY operated transfers the collector output of transistor QZA `from the timer input by way of the break contacts of contacts CY-.l to the base ot transistor QSA by way of the make contacts of contacts CY-l. With relay CON operated, the input of the timer is now connected to the collector of transistor Q3A by way `of lead liti?. and the make contacts of contacts CON-2, as previously described. The operation of relay CY removes the previously described negative marking clamp applied to the base of transistor Q3A by way of the break contacts of contacts CY-Z. Relay CY operated prepares an operating path for relay S which extends from the timer output lead 3% by way of the make contacts of contacts OR- and the make contacts of contacts CY-3 to the winding of relay S. Finally, the operation of relay CY completes an obvious operating path for relay MST, FIG. 2, by way of make contacts CY-l. Relay MST is the motor start relay and, when operated, functions to turn on the motor of teletypewriter 201 in the conventional manner.

The teletypewriter at the originating station is now ready for communication with the terminating station subject to bringing the teletypewriter motor up to normal speed. Communication will start when the terminating station responds by indicating that it is ready to receive a message. Since this occurs Iafter a one second interval, as described hereinafter, the attendant at the originating station proceeds to communicate with the terminating station upon receiving the answer-back signal.

Receiving a Call At the terminating station ringing current is applied over a path from the tip connection of the telephone line through the break contacts of contacts 0R42, the break contacts of contacts AN-12, ringer 115, and capacitor CB to the ring line connection. A portion of the ringing current will also tlow into a path in parallel with ringer M5 through capacitor (33C, thermistor RTlC, the winding of relay RU, in parallel with diode CRIC to the ring side of the line. The initial high resistance of thermistor RTllC prevents immediate operation of relay RU in order to avoid its yoperation on line surges. The ringing current heats thermistor RTlC gradually decreasing its resistance, until relay RU operates. As long as the ringing persists relay RU operates during each ringing interval and releases during each silent interval. Diode CRlC acts as a half-way rectifier to provide pulsating direct current to operate relay RU. Varistor RVlC, in shunt to thermistor RTlC and diode CRlC, protects the thermistor and diode from high vol-tage transients.

Assuming the terminating station is attended, the attendant thereat will answer the call by operating the answer key to close answer key contacts 406. This completes a path from ground by way of the break contacts of contacts S-3, the break contacts of contacts OR-3, answer key contacts 4%, the winding of relay AN, and the break contacts of contacts 8 9 to negative battery, operating relay AN, which locks by way of the make contacts of contacts AN-i. In the event that the station is unattended, lead 463 is strapped to the winding of relay AN whereby the operation of relay lRU provides ground by way of break contacts CY-7 and make contacts RU-11 operating relay AN.

Relay AN operated removes the previously described disabling ground applied to transistor QlA by way of the reak contacts `of contacts A-N-4. The operation of relay AN provides ground to answer lamp 407 by Iway of the break contacts of contacts S-3 and the make contacts of contacts AN-3. It is noted that ground is also supplied to answer lamp 467 by way of make contacts RU-S. The closure of the make contacts of contacts AN-12 completes the connection from the tip to the ring of the telephone line by way of the break contacts of contacts OR-12 and the make contacts of contact AN-IZ whereby the station goes off-hook. Relay AN operated removes the previously described negative disabling voltage applied to timer capacitor CSE by way of the break contacts of contacts AN-ll0 and provides a negative marking battery to the base of transistor QCiA by way `of the make contacts of contacts AN-lll. The operation of relay AN also tunes the oscillator to the F2 frequency by closing the make contacts of contacts AN-l and AN-Z and opening the break contacts of contacts A-N-l and AN-Z, as previously described. ln addition, relay AN removes the negative battery applied to timer capacitor CZE by Way of the break contacts of contacts AN-6, disconnects timer capacitor CSB lfrom the base of transistor QSE by way of break contacts AN-S, and extends capacitor CZE to the base of transistor QSE by way of the make contacts of contacts AN-o. As previously described, this arranges the timer to time the guard interval.

At the expiration of the guard interval, tnansistor QllE conducts, applying ground by way of lead 366-, the break contacts of contacts OR-, the make contacts of contacts AN-Q, and the break contacts of contacts M-S to the winding of relay M which operates and locks by way of the make contacts of contacts M-S and the make contacts of contacts AN-7. Relay M operated connects negative battery to the collector of transistor QZB by way of make contacts lvl-l2 whereby lfrequency P2M is transmitted to the line. Also, relay contacts M-ll and M-Z tune the discriminator to receive on the F1 band as previously described. The operation of relay M reconnects capacitor CSE to the base of transistor QiE by way of the make contacts of contacts M-land the break contacts of contacts S-'7. The timer at this time proceeds to monitor for the reception of a marking signal from the originating station. The operation of relay M also disconnects positive battery from the base of transistor Q21? thereby starting the automatic disconnect timing circuit as previously described. Finally, relay M operated extends timer output lead 396 to the winding of relay CON by way of the break contacts of contacts Oli-8, make contacts M-ltl, break contacts S- and break contacts of contacts CON-6.

With the terminating station transmitting the mar ing fnequency FEM, the originating station monitors the signal for the marking interval as previously described. At the conclusion ot the marking interval, the originating station transmits the mmking frequency FlM to the terminating station. Recalling that, after the operation of relay M at lthe terminating station, the timer is monitoring for the reception of the marking trequency, the reception of frequency FIM turns On transistor Qil. This supplies current by Way of lead 396 to the break contacts of contacts OR-S, make contacts M-ltl, break contacts S-l and the break contacts of contacts CON-6 to the winding tot relay CON, operating the relay which locks by way of the make contacts of contacts CONt` and the make contacts of contacts ANJ.

ln the event, however, that a telephone subscriber has dialed the station in error, frequency Fm is not received and relay CON does not operate. Accordingly, automatic disconnect transistor QZF is turned On, as previously described, operating relay S. Relay S operated opens the previously described enengizing path 4for the relay AN by way of the break contacts of contacts S-9 and extends the path by way tot the make contacts o contacts S-9, the break contacts of contacts CY-3, make contacts llvl-lt? and lead Slt-56 to the collec-tor of transistor QlE. Relay S operated also opens the previously described path connecting timing 'capacitor CBE to the base of transistor QSE via the break contacts of contacts S-'7. Accordingly, positive battery is applied to the base of transistor (23E through resistor R7E. Transistor QSE turns On, turning On, in turn, transistors QflI and QlE. The resultant ground at the emitter of transistor Q'lE is theretore applied by way of its collector to the winding of relay AN shunting the winding. The release et relay AN opens the previously described locking paths of relays M and S restoring the set to the initial quiescent condition.

Returning now to the operation of a relay CON, this prepares timer capacitor Cll to monitor for a one second spacing signal, connects the output of transistor QSA to the timer input, enables space-hold detector capacitor Cil-3D, completes the operating path for relay CY, disables timer capacitor `C31?, and completes a supplementary current path for the collector of oscillator transistor QZB in substantially the Sme manner as previously described for the originating station. ln addition, relay CGN operated reapplies battery to `the base of transistor QZF in the answer-back disconnect circuit by way of resistor Re? and make contacts CONJ, disabling the circuit. Finally, relay CON operated completes the previously described negative pulsing path for the answerback circuit by way of resistor RSF, make contacts lvl-l1 and CONdl, resistor Ria-F, and capacitor CMF to the base of transistor Qld? in the answer-back generator circuit. ln the event that the station is unattended, lead Ztl?) is strapped to diode CRiGlF whereby the teletypewriter character V is transmitted back to the originating station in response to the application of the negative pulse to the base of transistor Qlt'F as previously described.

Relay CY operated transfers the output of transistor QZA from. the timer input to the base of transistor QSA, removes the negative blinding signal applied to the base of transistor QSA, and completes the operating path for relay MST starting the teletypewriter motor, in substantially the same manner as previously described -for relay CY in the originating station.

The two teletypewriters are now ready for communication over the two frequency shift channels F1 and F2. Messages may now be transmitted in either direction until the communication interval is terminated by the disconnect and clearing sequence.

Disconnect and Clearing Sequence The disconnect and clearing sequence is initiated when communication is completed between the two stations. The sequence may be started at either station by operating the nonlocking clear key which includes normally open contacts dill, FIG. 4.

Assuming that the originating station operates the clear key, a lpath from ground is completed through the break contacts of contacts AN-7, the make contacts of contacts letl, contacts 1t-lll of the clear key, and the winding of relay S to battery, operating relay S which locks by way of the make contacts ot contacts S-6, the make contacts of contacts QRA, and the break contacts ot contacts AlN-7. The operation of relay S turns oit originating lamp 4% by opening the break contacts of contacts S43, and lights clear lamp 411i by extending ground to the lamp by way of the make contacts of contacts iS-. Relay S operated opens the previously described operating path `for relay CY by way of break contacts S-r, releasing relay CY. Relay S operated also opens the path extending `from the collector of the break transistor Qli-B to the base of transistor switch QEB by way of the break contacts of contacts S-S. This results in the application of positive battery to the base of transistor QSB by way of resistor RMB, turning Ol the transistor whereby the spacing frequency Pls is transmitted as previously described. This spacing frequency is used to disconnect the terminating station as described hereinafter. ln addition, relay S operated extends negative battery by way of resistor Rd, FIG. 3, the make contacts of contacts Si?, and make contacts CY-S to the base of transistor QSE in the timer circuit, applying negative battery to the base of transistor QSE during the momentary interval when relay S and relay CY are both operating. The application of the negative battery to the base 'of transistor QSE turns the transistor Ott thereby turning O5, in turn, transistors QSE and Q15, as previously described and discharges capacitor C1B.

When relay CY releases, the negative voltage applied to the base o transistor QSE by way of make contacts CY-S is removed, and capacitor C1B charges toward positive battery by Way of the make contacts of contacts CON-ll and resistor R7E. After about one second, the

voltage on the base of transistor QSE goes slightly positive, turning On the transistor which thereby turns On, in turn, transistors QZE and With transistor QlE con-ducting, current is applied by way or" the collectorto-emitter path of transistor QIlE, lead 396, the make contacts of contacts OR-S, the break contacts of contacts' CY-f3i, and the make contacts of contacts Se@ to the winding of relay OR.. This ground connection to the winding of relay OR short-circuits the relay and it releases. The release of relay CY also opens Ithe previously described operating path for relay MST and the teletypewriter motor is thereby turned Oil. in addition, the release of relay CY provides a negative marking signal to the base of transistor QSA by way of negative battery, resistor R2, the break contacts of contacts AN-ltl, the make contacts of contacts Gi-'Sg the break contacts of contacts CY-Z and resistor' RMA. The negative battery holds transistor QSA conductive, whereby a marking signal is applied to the keyer circuit by way off lead 466. Thus the teletypewriter selector magnet is maintained in the marking condition while the teletypewriter motor is slowing down to a stop, thereby preventing false characters from being stored in the teletypewriter.

As previously described, relay OR releases at the termination of the ione second interval. The release of relay OR removes the application of negative battery to capacitor C2 by Way of resistor R2, the break contacts of contacts AN-l-tl and the make contacts of contacts Oil-9. The potential of capacitor C2 is novv charged from positive battery by way of resistors RMA, llSA and RMA, and the break contacts of contacts CY-2. Accordingly, transistor QSA is maintained in the marking conductive condition until the potential to the base has risen suiiiciently, due to the charging of capacitor C2, to turn transistor Q3A. Off. This marking interval is maintained for a suilicient duration of time to prevent the printing of any false characters when the Iteletypewriter motor is turned on. Relay OR released also opens the previously described 'locking paths for relays CON and S by way of the make contacts of contacts O'Rdtl, whereby relays CON and S release. In addition, relay OR released places the originating station in the on-hook condition by opening the make contacts of contacts Oil-l2. Relay S released opens the previously described energizing path `for clear lamp dii, extinguishing Ithe lamp. Relay CON released opens the previously described collector current path for oscillator transistor QZB by way of make contacts CON-.l2 whereby transistor QZB is turned Off.

The termination of the disconnect and clearing sequence at the originating station has now resulted in the release of relays OR, S, and CON, as previously described above. This restores the originating station circuit to the initial condition.

As previously described, spacing frequency E1s is sent by the originating station during its clearing sequence to the terminating station. This prolonged spacing condition is detected at the terminating station by the discriminator, and applied by the collector of transistor QSA to the base .of timer transistor QSE by way of Ilead 592, as previously described. The resultant application of the spacing condition places negative battery on lead ed?. whereby transistor QSE turns On, applying a positive potential on the emitter of transistor QlE 'whereby diode CREE is back biased. As previously described, capacitor CiE proceeds to charge toward positive battery thereby turning On transistor QSE after an interval of approximately one second. When transistor QSE becomes conductive, transistor QZE and transistor QlE, in turn, become conductive. The resultant coilector-to-emitter current through transistor QlE is applied by way of lead 3%, the break contacts of contacts OR-S, the make contacts of contacts lit/4rd, the make contacts of contacts CY-S and the winding of relay S to battery, operating relay S which locks by way of the make contacts of contacts S-o and the make contacts of contacts AN7. Once relay S has operated at the terminating station, the action is essentially identical to that previously described at the originating station following the operation of relay S thereat, with the exception that the subsequent operation of the timer circuit applies the shunting ground to the fwinding of relay AN and relay OR in parallel thereto whereby relay AN is released at the terminating station. ln addition, the release of relay AN opens the previously described locking path for relay M releasing this latter relay.

ln the event that the terminating station initiates the disconnect sequence, the operation of the clear key completes a path from ground by Way of the make contacts of contacts AN-7, contacts el@ or" the clear key, and the winding of relay S to battery. The operation of relay S new completes the disconnect and clearing sequence at the terminating station in the same manner as previously described. lt is noted, however, that the terminating station, when cleared, sends the spacing lfrequency F25 toward the originating station when die collector of transistor Q4?, is disconnected from the base of transistor Q33 by way of the break contacts of contacts S-S. This spacing frequency is detected at the originating station, operating the timer circuit after an interval of approximateiy one second, whereby ground is applied by way of lead Silo, the make contacts of contacts OR-8 and the make contacts of contacts CY-S to the winding of relay S. The consequent operation of relay S provides the disconnect and clearing sequence at the originating station in the saine manner as previously described.

Transmission Failure Disconnect Interruption of die transmission of signals due to a transmission failure initiates the previously described disconnect and clearing sequence. Since, as previously described, the interruption of signals activates the space-hold detector, a continuous spacing condition is applied to the timer circuit. Consequently, after one second, a clearing and disconnect sequence will be initiated at the station which fails to receive signals. During this clearing sequence, the disconnecting station sends spacing frequency to the other station, as previously described. Both the stations thus disconnect in the same manner as previously described for the station which disconnects in response to the reception of the spacing frequency transmitted from the remote station during the disconnect sequence. Accordingly, a one-way transmission failure in either direction results in disconnection of both stations.

Local Operating M ode Operation of the locking local key connects the teletypewriter keyboard through a local circuit to the teletypewriter select magnet enabling the attendant to practice. Make contacts (i12 of the local key light ilocal lamp d3 when the local key is operated. In addition, the operation of the local key closes make contacts elle, completing a path from ground by way of contacts rtl4 and break contacts S-S to the winding of relay CY. The operation of relay CY, in turn, completes the operating path of relay MST, as previously described, whereby the teletypewriter motor is started. Sending contacts 262 and break contacts 2% of teletypewriter 2M transmit signals through break timer stage Qld-B, as previously described. The signals at the collector of transistor Q-iB are, in turn, applied by way of lead lll-fl and resistor RlSA to diode CRMA. lt is recalled that diode CRMA forms one of the three OR gate diodes feeding transistor stage QZA in the discriminator. Accordinoly, signals originating from the keyboard of teletypewriter 201 are fed by way of diode CRldA to the base of transistor QZ'A.

Iit is necessary to hold diodes CR9A and CREA nonconducting in order that there is no interference with the teletypewriter keyboard signals. Diode CRlllA is connected to the space-hold detector circuit, as previously described. When relays AN and OR are released, ground is applied by way of the break contacts of contacts AVN-7,

25 the break contacts or contacts Oli-i6, and lead 394 to the junction of resistors R151) and in the space-hold detector circuit, thereby shouting the space-hold negative bias voltage supply. Accordingly, this shunt is effected in the local mode in order to render diode ClllA nonconducting.

Diode CRQA is connected to the emitter of transistor QlA. The base of transistor QlA is `fed by the output `ot the discriminator and is connected to the junction or inductor L1A and capacitor (27A. When relays AN and OR are released, shunting ground is applied by way of the break contacts of contacts AN-l, the break contacts of contacts OR-2, and lead 'dilo to the base o transistor QlA, maintaining the emitter of transistor QlA at about ground potential. With the emitter of transistor QlA at ground, and diode ClA connected by Way of resistors RQA, R6A and RltA to positive battery, diode Cl9A is reversely biased and, therefore, nonconducting.

The signals provided to the base of transistor QZA by way of diode CRldA appear at the collector of transistor (22A and are then applied by Way of transistor QSA, lead 493, and the keyer circuit to select magnet 265 of teletypewriter Ztll, as previously described. Thus, signals transmitted through send contacts 282 are applied to select vmagnet fait. At the conclusion of 'the practice period, the station is restored -to normal by operating the olear key which mechanically releases the locked local key. The local ltey released opens the operating path tor relay CY, whereby relay MST is released and the station is restored to the initial condition.

lt is noted that the data set accepts incoming calls While in the local mode it the attendant operates the answer lrey when ringer ll is activated.

Test Operating Mode The test mode `is provided to enable certain performance tests to be made by a remote test center. These tests are performed by looping the demodulator output signals through transistor stages QSE and Qd-E of the timer circuit to the input of the frequency shift oscillator. Assuming that the station has originated a call to the test center and has been connected in the normal manner, relays OR, `CGN and CY are operated The attendant operates the test key in the data set completing an obvious energi ing path for test lamp lll by way of `malte contacts il@ of the test ltey. Malte contacts llo also provide ground oy way of diode CRlB to the emitter of transistor Q-t-B, thereby permitting signals originating from the teletypewriter keyboard to be applied by Way of lead lll to diode CENA. As previously described, the signels applied -to diode CRlrlA are passed through transistor QZA and transistor Q3A to the collector of transistor QSA. With relay CON operated, the output of transistor QEIA is applied by AWay of lead dit?. to the base of transistor QSE in the timer. The signals are thus transmitted by way of transistors QSE and QLE to the junction of resistors Ell-E and RSE connected to the emitter of transistor Qdl. These signals are then applied by way of resistor RlZE, lead 369, the make contacts of test liey contacts lilo, and the break contacts of contacts S-S to the base of transistor (23B. Accordingly, with the test key operated, signals received from the test center are applied by Way of the limiter and the discriminator to transistors (25E and Qtl in the timer, and by Way of lead 3G@ to the oscillator, whereby the signals are retransmitted back to the test center. `ln addition, with the test center transmitting continuous marking signals, signals may be transmitted from send contacts 292 to the remote test center by way of transistor QLB, lead ll, transistors QZA and (23A, 'lead llt, transistors QSE and (24E, lead 309, and the frequency shift oscillator. lt is noted that the output of transistor Q3A also applies the signals to select magnet 26S by way of the lieyer circuit, as previously described. rfhus, a test message transmitted CII F2 virctpaency band will be repeated in the F1 frequency band back to the test center and, at the same time, the teletypen/riter receiver will monitor the testmessage. This test message may be also re eived `in the l?, frequency band and transmitted baclc in the frequency band by placing the subset in the terminating mode.

Intero/ranging Frequency Bonds Under certain conditions it may be desirable to interchange the frequency band `without disconnecting from the remote station or test center. Assuming that the subset is in the originating mode, the clear key is momentarily operated and the answer key is operated immediately thereafter and held operated until answer lamp 467 lights.

The closure of contacts llt? of the clear key operates relay S to initiate the disconnect and clearing sequence, as previously described. During the disconnect and clearing sequence, relays CY and MST release, followed oy the release of relays OR, S and CON, as previously described. The release of relays R and S with the answer key operated completes a path from ground by way of the break contacts ot contacts S-, the break contacts ot contacts @l-3 and make contacts dilo of the answer lcey to the winding of relay AN. Accordingly, either the malte contacts of contacts Oil-l2 or the `mal-:e contacts of contacts AlN-l2 are closed during the switching process except for a brief transition interval of a efe-w milliseconds so that the oilhookY condition of the telephone line Will be maintained at the local station. Operation of relay AN now results in placing the data set in the terminating snode in the same manner las previously described. rlhe remote station, however, must transmit marking frequency FIM to complete the sequence, placing the data set in the terminating mode. Signals may then be transmitted from the remote station or test center to the local data set in the F1 frequency band and signals from the local data set are transmitted in the F2 frequency band.

Although a specific embodiment of the invention has been shown and described, it will be understood that various modifications may be made without departing yfrom the spirit of the invention and within the scope of the appended claims.

What is claimed is:

=l. ln a data tnansmission system including a two-way transmission channel capable of conveying ringing and data signals, signaling means for applying signals to said channel, receiving means responsive to data signals from said channel, inverter means responsive Ito said receiving means, timing means `operable Ia predtermined interval after energization thereof, operable means responsiveto the reception of ringing signals from said channel for energizing said timing means in a first instance, means responsive to said loperation of said tinning means in said lirst instance for operating said signalling moans and rendering said timing means responsive to said receiving moans in `a second instance, and means respons-ive to said operation of said timing means in said second instance for rendering said timing means `responsive to said inverter means in la `third instance.

2. ln `a data tnansmission system including a two-Way transmission channel, signaling means for applying signals to said channel, receiving means responsive to data signals from said channel, inverter means responsive to said 'receiving means, timing means operable a predetermined interval after energization thereof, operable means for energizing said timing means in a irst instance, means responsive to said operation of said timing means in said iirst instance for operating said signaling means and rendering said timing means responsive to said receiving means in a second instance, means respons-ive to said operation of said timing means in said second instance for rendering said timing means responsive to said inventer means in a third instance, and means re- 2'? sponsive to said operation of said timing means in said third instance for disabling said receiving means.

3. `In a data transmission system including a two-Way transmission channel, receiving means responsive to data signals from said channel, recording means, inverter means for rapplying dat-a signals to said recording means, timing means operable a predetermined interval after energization thereof, operable means for energizing said timing means in a rst instance, means responsive to said operation of said tim-ing means in said rst instance for extending the output of said receiving means to said timing means lin a second instance, means responsive to said operation of said timing means in said second instance for transferring said output of said receiving means to said inverter means and extending said output of said inverter means to said timing means 'in Ia third instance, and means responsive to said operation of said timing means in said third instance for rendering said receiving means nonresponsive to signals from said channel.

4. ln a data transmission system including a transmission clhannel capable of conveying data signals having a first or second condition, rece'iving means for receiving said data signals from said channel, inverter means for reversing the condition of said signals, recording means responsive to said inverter means for recording said signals, timing means operable in response to the application of said first condition signal, means for normally applying the output of said receiving means to said timing means, operable means for extending said channel to said receiving means, means responsive to the operation of said timing means for transferring said output of said receiving means to said inverter means and applying the output of said inverter means to said timing means, and means thereafter responsive to the operation of said timing means for disabling said operable means.

5. In ya data transmission system including la transmission channel capable of conveying signals having a lirst or second condi-tion, means operable in response to the application of said first condition signal, receiving means for applying signals received from said channel to said operable means, inverter means for reversing the condition of said signals, and means responsive to the operation of said operable means for transferring the output of said receiving means to said inverter means and applying the output of said inverter means to said timing means.

6. In a telegraph system including a transmission chennel for transmitting mark-ing and spacing telegraph signals, means connected to said channel for generating first and second condition signals in accordance with the reception of said marking and spacing signals from said channel, inverter means for reversing the condition of said tirst and second condition signals, means operable in response to the generation of said first condition signal by said generating means, and means responsive to the operation of said operable means for rendering said operable means responsive to said inverter means and extending the output of said generating means to said inverter means.

7. In la telegraph station including a two-Way transmission channel capable of conveying telegraph signals, la normally disabled telegraph mechanism, an inverter circuit for applying telegraph signals to said mechanism, means for receiving said telegraph signals, signaling means for `applying signals to said channel, operable means for extending said channel to said receiving means, timing means normally connected to the output of said receiving means, other means responsive to said timing means for transferring the output of said receiving means from said timing means to said inverter circuit and extending the output of said inverter circuit to said timing means, means responsive to said other means for enabling said telegraph mechanism, and means thereafter responsive to said timing means for operating said signaling means and releasing said operable means.

8. A data transmission system including a two-Way transmission channel comprising a recorder, an inverter circuit for applying data signals to said recorder, timing leans operable a predetermined interval `after the application of a signal thereto, receiving means ior applying data signals received from said channel to said timing means, Kand other means responsive to said operation of said timing means for transferring the output of said receiving means from said timing means to said inverter circuit 'and extending the output of said inverter circuit to said timing means.

9. ln a telegraph station including a two-Way transmission channel capable of conveying telegraph signals, a telegraph mechanism, an inverter circuit for applying telegraph signals to said mechanism, means for receiving said telegraph signals, signaling means for applying signals to said channel, operable means for extending said channel to said receiving means, timing means normally connected to the output of said receiving means, means responsive to said timing means for operating said sigmaling means, `other means responsive to said timing means for transferring the output of said receiving means from said timing means to said inverter circuit and extending the output of said inverter circuit to said timing means, and means thereafter responsve to said timing means for releasing said operable means.

10. yln a telegraph station including a two-way transmission channel capable of conveying ringing and telegraph signals, a normally disabled telegraph mechanism, an inverter circuit for applying telegraph signals to said mechanism, means for receiving said telegraph signals, signaling means for applying signals to said channel, operable means responsive to the reception of said ringing signals for extending said channel to said receiving means, timing means normally connected to the output of said receiving means, other means responsive to said timing means for transferring the output of said receiving means from said timing means to said inverter circuit and extendng the output of said inverter circuit to said timing means, means responsive to said other means for enabling said telegraph mechanism, and means thereafter responsive to said timing means for operating said signaling means and releasing said operable means.

11. A telegraph station including a two-Way transmission channel capable of conveying ringing and telegraph signals comprising a normally disabled telegraph mechanism, ari inverter circuit for applying telegraph signals to said mechanism, signaling means for applying signals to said channel, timing means operable a predetermined interval after the application of a signal thereto operable means responsive to ringing signals on said line for enabling said timing means, receiving means responsive to said operable means for applying sign-als received from said channel to said timing means, other means responsive to said operation of said timing means for transferring the output of said receiving means for said timing means to said inverter circuit and extending the output of said inverter circuit to said timing means, means responsive to said other means for enabling said mechanism, and means thereafter responsive to said operation of said timing means for operating said signaling means and releasing said operable means.

l2. ln a data transmission system including a two-way transmission channel, iirst signaling means for applying a signal of a iirst frequency to said channel, second si-gnaling means for applying a signal of a second frequency to said channel, irst receiving means responsive to the reception of said first frequency signal from said channel, second receiving means responsive to the reception of said second frequency signal from said channel, rst operable means for enabling said iirst receiving means and said second signaling means, second operable means for enabling said second receiving means and conditioning said first signaling means, and means enabled by said responsive second receiving means for enabling said conditioned first signaling means.

13. `In a d-ata transmission system including a two-way ltransmission channel, first signaling means `for applying a signal of a first frequency to said channel, said signaling means for applying a signal yof a `second `frequency tosaid channel, irst receiving means lresponsive to the reception of said irst frequency signal from said channel, second receiving means responsive to the reception of said second frequency sign-a1 from said channel, signal recording means, first :operable means for enabling said first receiving means and said second signaling means, second 10 operable means for enabling said second receiving means and conditioning said iirst signaling means, means enabled by said responsive first receiving means for rendering said signal recording means responsive to said first ,receiving means, and means enabled =by said responsive second receiving means for enabling said conditioned first signaling means and rendering said signal recording means responsive to said second receiving means.

No references cited. 

1. IN A DATA TRANSMISSION SYSTEM INCLUDING A TWO-WAY TRANSMISSION CHANNEL CAPABLE OF CONVEYING RINGING AND DATA SIGNALS, SIGNALING MEANS FOR APPLYING SIGNALS TO SAID CHANNEL, RECEIVING MEANS RESPONSIVE TO DATA SIGNALS FROM SAID CHANNEL, INVERTER MEANS RESPONSIVE TO SAID RECEIVING MEANS, TIMING MEANS OPERABLE A PREDETERMINED INTERVAL AFTER ENERGIZATION THEREOF, OPERABLE MEANS RESPONSIVE TO THE RECEPTION OF RINGING SIGNALS FROM SAID CHANNEL FOR ENERGIZING SAID TIMING MEANS IN A FIRST INSTANCE, MEANS RESPONSIVE TO SAID OPERATION OF SAID TIMING MEANS IN SAID FIRST INSTANCE FOR OPERATING SAID SIGNALING MEANS AND RENDERING SAID TIMING MEANS RESPONSIVE TO SAID RECEIVING MEANS IN A SECOND INSTANCE, AND MEANS RESPONSIVE TO SAID OPERATION OF SAID TIMING MEANS IN SAID SECOND INSTANCE FOR RENDERING SAID TIMING MEANS RESPONSIVE TO SAID INVERTER MEANS IN A THIRD INSTANCE. 